Mode-locked long fibre master oscillator with
intra-cavity power management and pulse
energy > 12 µJ
Alexey Ivanenko,1,* Sergey Kobtsev,1,2 Sergey Smirnov,1 and Anna Kemmer1
1
Division of Laser Physics and Innovative Technologies, Novosibirsk State University, Pirogova str., 2, Novosibirsk,
630090, Russia
2
”Tekhnoscan-Lab” LLC, Inzhenernaia str., 26, Novosibirsk, 630090, Russia
*
[email protected]
Abstract: Combined lengthening of the cavity of a passive mode-locked
fibre master oscillator and implementation of a new concept of intra-cavity
power management led to achievement of a record-high pulse energy
directly at the output of the mode-locked fibre master oscillator (without
any subsequent amplification) exceeding 12 µJ. Output powers at the level
of > 12 µJ obtainable from a long-cavity mode-locked fibre master
oscillator open new possibilities of application of all pulse types that can be
generated in such oscillators.
©2016 Optical Society of America
OCIS codes: (140.0140) Lasers and laser optics; (060.3510) Lasers, fiber; (140.4050) Modelocked lasers; (140.3615) Lasers, ytterbium; (140.3410) Laser resonators; (140.7090) Ultrafast
lasers.
References and links
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P. Grelu and N. Akhmediev, “Dissipative solitons for mode-locked lasers,” Nat. Photonics 6(2), 84–92 (2012).
S. Kobtsev, S. Kukarin, and Y. Fedotov, “Ultra-low repetition rate mode-locked fiber laser with high-energy
pulses,” Opt. Express 16(26), 21936–21941 (2008).
B. N. Nyushkov, V. I. Denisov, S. M. Kobtsev, V. S. Pivtsov, N. A. Kolyada, A. V. Ivanenko, and S. K.
Turitsyn, “Generation of 1.7-uJ pulses at 1.55um by a self-modelocked all-fiber laser with a kilometers-long
linear-ring cavity,” Laser Phys. Lett. 7(9), 661–665 (2010).
S. M. Kobtsev, S. V. Kukarin, S. V. Smirnov, and Y. S. Fedotov, “High-energy mode-locked all-fiber laser with
ultralong resonator,” Laser Phys. 20(2), 351–356 (2010).
A. Ivanenko, S. Turitsyn, S. Kobsev, and M. Dubov, “Mode-locking in 25-km fiber laser,” European Conference
on Optical Communication (ECOC), 1–3, (2010).
S. V. Smirnov, S. M. Kobtsev, S. V. Kukarin, and S. K. Turitsyn, Mode-Locked Fibre Lasers with High-Energy
Pulses (InTech, 2011).
C. Aguergaray, A. Runge, M. Erkintalo, and N. G. R. Broderick, “Raman-driven destabilization of mode-locked
long cavity fiber lasers: fundamental limitations to energy scalability,” Opt. Lett. 38(15), 2644–2646 (2013).
M. E. Fermann, F. Haberl, M. Hofer, and H. Hochreiter, “Nonlinear amplifying loop mirror,” Opt. Lett. 15(13),
752–754 (1990).
E. J. R. Kelleher and J. C. Travers, “Chirped pulse formation dynamics in ultra-long mode-locked fiber lasers,”
Opt. Lett. 39(6), 1398–1401 (2014).
E. Ding, W. H. Renninger, F. W. Wise, P. Grelu, E. Shlizerman, and J. N. Kutz, “High-energy passive modelocking of fiber lasers,” Int. J. Opt. 2012, 354156 (2012).
R. I. Woodward, E. J. R. Kelleher, T. H. Runcorn, S. Loranger, D. Popa, V. J. Wittwer, A. C. Ferrari, S. V.
Popov, R. Kashyap, and J. R. Taylor, “Fiber grating compression of giant-chirped nanosecond pulses from an
ultra-long nanotube mode-locked fiber laser,” Opt. Lett. 40(3), 387–390 (2015).
S. Smirnov, S. Kobtsev, S. Kukarin, and A. Ivanenko, “Three key regimes of single pulse generation per round
trip of all-normal-dispersion fiber lasers mode-locked with nonlinear polarization rotation,” Opt. Express 20(24),
27447–27453 (2012).
S. Kobtsev, S. Kukarin, S. Smirnov, S. Turitsyn, and A. Latkin, “Generation of double-scale femto/pico-second
optical lumps in mode-locked fiber lasers,” Opt. Express 17(23), 20707–20713 (2009).
B. Nie, G. Parker, V. V. Lozovoy, and M. Dantus, “Energy scaling of Yb fiber oscillator producing clusters of
femtosecond pulses,” Opt. Eng. 53(5), 051505 (2013).
S. V. Smirnov, S. M. Kobtsev, and S. V. Kukarin, “Efficiency of non-linear frequency conversion of doublescale pico-femtosecond pulses of passively mode-locked fiber laser,” Opt. Express 22(1), 1058–1064 (2014).
S. Kobtsev, S. Kukarin, S. Smirnov, and I. Ankudinov, “Cascaded SRS of single- and double-scale fiber laser
pulses in long extra-cavity fiber,” Opt. Express 22(17), 20770–20775 (2014).
#255592
(C) 2016 OSA
Received 11 Dec 2015; revised 12 Feb 2016; accepted 23 Feb 2016; published 17 Mar 2016
21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.006650 | OPTICS EXPRESS 6650
17. S. M. Kobtsev, S. V. Kukarin, and Y. S. Fedotov, “High-energy Q-switched fiber laser based on the sidepumped active fiber,” Laser Phys. 18(11), 1230–1233 (2008).
18. S. Kobtsev, S. Smirnov, S. Kukarin, and S. Turitsyn, “Mode-locked fiber lasers with significant variability of
generation regimes,” Opt. Fiber Technol. 20(6), 615–620 (2014).
19. Y. S. Fedotov, A. V. Ivanenko, S. M. Kobtsev, and S. V. Smirnov, “High average power mode-locked figureeight Yb fibre master oscillator,” Opt. Express 22(25), 31379–31386 (2014).
20. B. G. Bale, J. N. Kutz, A. Chong, W. H. Renninger, and F. W. Wise, “Spectral filtering for mode locking in the
normal dispersive regime,” Opt. Lett. 33(9), 941–943 (2008).
21. A. B. Grudinin, D. N. Payne, P. W. Turner, L. J. A. Nilson, M. N. Zervas, M. Ibsen, and M. K. Durkin, “Multifibre arrangements for high power fibre lasers and amplifiers,” United States patent 6,826,335 B1, (November
30), 2004.
22. V. J. Matsas, T. P. Newson, D. J. Richardson, and D. N. Payne, “Self-starting passively mode-locked fibre ring
soliton laser exploiting nonlinear polarisation rotation,” Electron. Lett. 28(15), 1391–1393 (1992).
23. C. Vinegoni, M. Wegmuller, B. Huttner, and N. Gisin, “Measurement of nonlinear polarization rotation in a
highly birefringent optical fibre using a Faraday mirror,” J. Opt. A, Pure Appl. Opt. 2(4), 314–318 (2000).
24. M. Rusu, R. Herda, S. Kivistö, and O. G. Okhotnikov, “Fiber taper for dispersion management in a mode-locked
ytterbium fiber laser,” Opt. Lett. 31(15), 2257–2259 (2006).
25. A. Chong, W. H. Renninger, and F. W. Wise, “Observation of antisymmetric dispersion-managed solitons in a
mode-locked laser,” Opt. Lett. 33(15), 1717–1719 (2008).
26. F. Ö. Ilday and F. W. Wise, “Nonlinearity management: a route to high-energy soliton fiber lasers,” J. Opt. Soc.
Am. B 19(3), 470–476 (2002).
27. R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Ybdoped fibres,” Opt. Commun. 136(5–6), 375–378 (1997).
1. Introduction
The principle of considerably raising the pulse energy of passive mode-locked fibre master
oscillators (‘dissipative soliton fibre lasers’ [1]) due to greatly lengthened oscillator cavity [2]
has resulted in relatively high pulse energies in the vicinity of several µJ directly at the output
of the master oscillator at the resonator length in the range of 1–25 km [3–6]. Various master
oscillator configurations were tested having both normal and anomalous total cavity
dispersion. Nevertheless, in none of the studied cavity layouts the output pulse energy
exceeded at most few µJ. In [2], the obtained pulse energy amounted to 3.9 µJ at the cavity
length of 3.8 km, and subsequent attempts at raising the output pulse energy further by
elongation of the cavity first to 8 km [4] and then to 25 km [5] only led to proportionally
lower average radiation power at which a stable mode-locked operation was still possible.
The average radiation power had to be reduced because higher peak pulse power in longer
resonators resulted in unstable mode locking due to nonlinear effects, in particular Ramandriven destabilisation [7]. Therefore, the pulse energy rose with the resonator length up to a
certain limit value (4 µJ [4,5]), after which it saturated: longer cavities required lower average
radiation power for stable mode locking. Consequently, the maximal achievable pulse energy
in a passive mode-locked long fibre oscillator was capped at 4 µJ and stayed at this level for
approximately the last 7 years [2].
The present work for the first time breaks this limit and reports pulse energy exceeding 12
µJ directly at the output of a mode-locked Yb-based long fibre oscillator. This significant rise
in pulse energy became possible owing to a new approach to cavity design of fibre master
oscillators: we implemented a specially crafted layout, which distributed the intra-cavity
radiation so as to maintain lower radiation power in the long part of the cavity, thus
minimising nonlinear effects in the long fibre. Additionally, instead of the previously used
mode locking mechanism based on nonlinear polarisation evolution (NPE), in this work we
relied on a nonlinear amplifying loop mirror (NALM) [8]. Combining these two new
approaches to design of passive mode-locked long fibre master oscillators resulted in pulses
with energy of 12.4 µJ at the output of the oscillator.
It must be noted that the duration of pulses generated in long cavities of mode-locked
fibre master oscillators falls into the nanosecond range, while the pulse repetition rate in the
fundamental mode locking regime amounts to tens or hundreds of kHz. Pulse parameters of
mode-locked long fibre master oscillators are close to those of Q-switched fibre lasers.
Nevertheless, pulses generated in these long mode-locked lasers possess certain distinctive
features. For example, long fibre master oscillators mode-locked due to NPE or by nanotubes
#255592
(C) 2016 OSA
Received 11 Dec 2015; revised 12 Feb 2016; accepted 23 Feb 2016; published 17 Mar 2016
21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.006650 | OPTICS EXPRESS 6651
produce pulses, which may exhibit giant chirp [9,10] and may be consequently compressed
by a factor of up to ~100 [11] or at least to a fraction of their initial duration in the case of
transient pulses [12]. Transient and noise-like (‘double-scale’ [13]) pulses may be also very
interesting for applications due to their specific internal structure [14] and high peak power
which results in high efficiency of nonlinear transformations [15,16].
Another salient feature of pulses generated in mode-locked regimes is that their repetition
rate does not depend on the radiation power, unlike that of pulses generated in passive Qswitched fibre lasers [17]. In the majority of implementations, duration of pulses generated in
passive mode-locked fibre lasers depends much less on the radiation power than it does in
passive Q-switched fibre lasers [17,18].
Therefore, pulses produced in passive mode-locked fibre master oscillators exhibit certain
peculiarities, which may be useful in various applications. Higher available energy of such
pulses is in demand and effectively broadens the area of their potential applications.
2. Experiment
The experimental set-up used in the present work is schematically shown in Fig. 1.
Fig. 1. Schematic diagram of the experimental set-up: LD – pumping laser diode, PC – fibre
polarisation controller, PBS – fibre polarisation beam splitter, FM – fibre Faraday mirror, IS –
fibre isolator, d1 and d2 – PM fibres.
The laser cavity is composed of a linear and a ring-shaped arms, the latter one laid out as a
NALM and containing non-polarisation-maintaining fibre and two stretches of polarisationmaintaining fibre (d1 = 80 cm, d2 = 90 cm) which, together with the fibre-based polarisation
controller, form a fibre birefringent spectral filter [19]. As it was earlier demonstrated in [20],
spectral filtration of radiation in a normal-dispersion fibre laser cavity improves stability of
passive mode-locked lasing, allowing generation of pulses with greater energy. The gain
section of NALM includes two (7- and 9-m long) active side-pumped Yb-doped fibres
(GTWave fibre design explained in [21]) each with its independent pump. Their absorption of
the pump radiation at 980 nm was 3.9 dB/m. The 7-m long fibre was pumped with 3.5 W,
while the other one received 4.5 W. Adjustment of the pump intensity ratio makes it possible
to control the phase incursion difference of the two counter-propagating waves in the NALM.
This adjustment not only ensures stable operation of the laser at different output radiation
power levels, but also allows to trigger mode locking at relatively low generated radiation
power. The ring section of the cavity is connected to the linear one through a polarisationmaintaining (PM) fibre coupler with 40/60 ratio (the 40% port is used to couple the radiation
out of the laser).
The linear arm of the laser cavity consists of two direction-split linear segments placed
between two fibre polarisation beam splitters, 2.5-km fibre stretch for cavity elongation, and a
Faraday mirror at its opposite end.
Mode-locked operation is triggered at a certain pumping power ratio between the two
active fibres in the NALM, the total (both sources) pumping power threshold amounting to 3
W. The polarisation controller is not used for starting mode-locked operation, its function
#255592
(C) 2016 OSA
Received 11 Dec 2015; revised 12 Feb 2016; accepted 23 Feb 2016; published 17 Mar 2016
21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.006650 | OPTICS EXPRESS 6652
being only to control the transmission bandwidth of the fibre-based birefringent filter.
Although the cavity contains non-PM fibre spans, polarisation-sensitive elements and a
polarisation controller, we believe that our laser is not appreciably affected by nonlinear
polarisation evolution [22] because after mode-locked operation is established, it cannot be
disrupted by adjusting the polarisation controller settings.
After mode locking is established, radiation pulses move along the cavity in the following
manner: upon exit from the NALM, they are fed through the 60% port of the PM coupler into
the split section of the linear cavity arm where they are direction-split at PBS1 and once again
combined at PBS2. The propagation direction of radiation in the split section of the cavity is
determined by optical isolators IS1 and IS2. The upper branch (Fig. 1) is taken by pulses
going away from the NALM, while the lower branch is used to pass those moving towards
the NALM. The split section of the linear cavity arm with a variable attenuator in the upper
arm is used to lower radiation power in a 2.5-km-long fiber section. Correspondingly, the
power of the radiation passing through the lengthening fibre becomes several times as low as
that of the radiation inside the NALM, thus considerably reducing nonlinear effects in the
cavity-lengthening fibre. A Faraday mirror at the end of the cavity’s linear segment [23]
serves to eliminate all the fluctuations in the linear birefringence of the cavity-lengthening
fibre, including temperature- and pressure-induced ones. Upon the return pass through the
2.5-km fibre, the radiation pulses are fed into the lower branch of the split section, which
contains one of the pumped active fibres. This restores the radiation power to the level it had
upon exit from the NALM.
Therefore, the proposed laser resonator establishes two zones with significantly different
intra-cavity radiation power: NALM zone with relatively high intra-cavity radiation power
and the cavity-lengthening zone with relatively low radiation power. The NALM zone
ensures high output radiation power, while the cavity lengthening zone reduces the pulse
repetition rate. As a result, this composite-cavity master oscillator produces output pulses
with relatively low repetition rate (41.7 kHz) featuring record-high energy of 12.4 µJ (which
corresponds to the laser’s average output power of 520 mW).
By analogy to dispersion management [24,25] and nonlinearity management [26], the
proposed laser configuration for the first time implements intra-cavity power management
allowing different intra-cavity radiation power in different parts of the laser cavity.
Figure 2 demonstrates the temporal envelope of the generated pulses recorded at 40-ps
resolution with a photo-detector and a storage oscilloscope. The measured pulse duration was
4.3 ns, the central generation wavelength 1078.9 nm, and the spectrum FWHM (Fig. 2 c) was
0.06 nm without any sidebands. Spectrum width of 0.06 nm corresponds to coherence time of
25–28 ps, which is unusually long for stochastic filling of double-scale pulses. However, the
question about internal structure of such pulses and their compressibility will be the subject of
separate study.
It must be pointed out that the proposed laser layout requires only two pump sources for
its operation: the first one in the NALM and the second one between the NALM and the
cavity-lengthening fibre. The first of these sources ensures proper functioning of the NALM,
while the second provides intra-cavity power management.
Let’s estimate how the proposed intra-cavity power management solution opens a new
way for pulse energy scaling. As the laser cavity length L increases, an approximately linear
corresponding growth of the duration of the generated pulses T may be expected due to
increasing laser cavity dispersion, T ~L. It is necessary to limit nonlinear effects in order to
ensure stable mode locking: γP(z) dz ≤ φmax, where P(z) corresponds to the distribution of the
radiation power along the cavity, γ – nonlinear coefficient, φmax – critical nonlinear phase
incursion. This limitation suggests that the power of the generated pulses P should drop
inversely proportional to the cavity length: P ~1/L.
#255592
(C) 2016 OSA
Received 11 Dec 2015; revised 12 Feb 2016; accepted 23 Feb 2016; published 17 Mar 2016
21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.006650 | OPTICS EXPRESS 6653
Fig. 2. Pulse operation of the fibre master oscillator. (a) Temporal pulse shape. (b)
Oscilloscope trace of the typical laser output. (c) Pulse spectrum measured.
Therefore, we have the value of the pulse energy: W ~T × P ~L × L–1 ~const, which is
what was actually observed during the preceding experiments [2–5]. The new configuration
proposed in the present work consists of two arms with different length (L1 << L2) having
different radiation power level inside them (P1 >> P2). Consequently, for nonlinear phase
incursion we obtain: φNL = γ1P1(z) dz + γ2P2(z) dz ≤ φmax. For a basic estimate, let’s take γ1 =
γ2, P1(z) = P = const, P2 = P/a, where a is the coefficient of signal attenuation within the
second arm of the cavity. Then, we have φNL = γP × (L1 + L2/a) ≤ φmax. Therefore, we can
derive the expression for pulse energy scaling due to the second fibre segment (with low
nonlinearity) added to the cavity:
L + L2
W
PT
≈
≈ 1
(1)
W0 P0T0 L1 + L2 a
Coefficient kW represents the ratio of energy W of pulses generated in the cavity
containing a lengthening fibre L2, to the corresponding energy W0 observed in a cavity of
length L1. At L2 = 0 or a = 1 we have kW = 1. In the limiting case of L2 >> L1 and L2 / a >> L1,
we obtain kW ~a. This means that the proposed intra-cavity power management approach in
long cavities of mode-locked fibre master oscillators leads to enhancement of the output pulse
energy in the vicinity of the value of the attenuation of the radiation power within the cavitylengthening fibre L2. Thus, output pulse energy in significantly lengthened cavities of modelocked fibre lasers may be enhanced by roughly the same factor as the energy inside the
cavity-lengthening fibre is reduced.
There are two possible fundamental limitations of output energy enhancement by intracavity power management in passive mode-locked long fibre master oscillators:
kW =
#255592
(C) 2016 OSA
Received 11 Dec 2015; revised 12 Feb 2016; accepted 23 Feb 2016; published 17 Mar 2016
21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.006650 | OPTICS EXPRESS 6654
а) substantial absorption of radiation in the cavity-lengthening fibre;
b) pulse repetition period reaching values comparable to the upper state lifetime of the
active medium of the laser (~1 ms for ytterbium-doped silica fibres [27]).
In the studied laser, the pulse repetition period amounted to 24 µs. Accordingly, it is likely
possible to slow down the pulse repetition by more than an order of magnitude before
limitation b) kicks in. Such cavity elongation (by more than an order of magnitude) is also
possible if the cavity-lengthening fibre with relatively low losses at the generation wavelength
is chosen (e.g. with attenuation not exceeding 0.5 dB/km). In such a case, it is not
unreasonable to expect pulse energies at the output of a passive mode-locked long fibre
master oscillator reaching 100 µJ and even higher. One of the probable technical obstacles to
achievement of ultra-high pulse energies directly at the output of a fibre master oscillator is
limited radiation damage threshold of the employed fibre components.
3. Conclusion
The present work for the first time demonstrates that record-high pulse energies exceeding 12
µJ can be obtained directly at the output of passive mode-locked Yb-based long fibre
oscillator without resorting to additional fibre amplifiers. Mode locking was implemented
with the help of a nonlinear amplifying loop mirror, while nonlinear effects in a 2.5-km
cavity-lengthening fibre were substantially reduced through a specially designed master
oscillator layout, in which intra-cavity radiation power may be significantly different in
different parts of the cavity. The developed layout using an intra-cavity power management
solution, which is proposed here for the first time, opens prospects of further pulse energy
enhancement in passive mode-locked long fibre master oscillators (reaching 100 µJ and
higher), which was only limited in the present work by the available pump power and the
length of the used fibre.
Acknowledgments
This work was supported by the Grants of Ministry of Education and Science of the Russian
Federation (agreement No. 14.B25.31.0003, ZN-06-14/2419, order No. 3.162.2014/K);
Project 5-100 funding (NSU, СЗ4.2); the Russian Foundation for Basic Research (agreement
No. 16-32-60160, 16-02-00104); Foundation for Promotion of Small Enterprises in Science
and Technology (agreement No. 138AGR/18581).
#255592
(C) 2016 OSA
Received 11 Dec 2015; revised 12 Feb 2016; accepted 23 Feb 2016; published 17 Mar 2016
21 Mar 2016 | Vol. 24, No. 6 | DOI:10.1364/OE.24.006650 | OPTICS EXPRESS 6655